Planar lightwave circuits comprise fundamental building blocks for the newly emerging, modern fiberoptic communications infrastructure. Planar lightwave circuits are innovative devices configured to transmit light in a manner analogous to the transmission of electrical currents in printed circuit boards and integrated circuit devices. Examples include arrayed waveguide grating devices, integrated wavelength multiplexers/demultiplexers, optical switches, optical modulators, wavelength-independent optical couplers, and the like.
Planar lightwave circuits (PLCs) generally involve the provisioning of a series of embedded optical waveguides upon a semiconductor substrate (e.g., silicon), with the optical waveguides fabricated from one or more silica glass layers, formed on an underlying semiconductor substrate. PLCs are constructed with a number of waveguides precisely fabricated and laid out across a silicon wafer. A conventional optical waveguide comprises an un-doped silica bottom clad layer, with at least one waveguide core formed thereon, and a cladding layer covering the waveguide core, wherein a certain amount of at least one dopant is added to both the waveguide core and the cladding layer so that the refractive index of the waveguide core is higher than that of the cladding layer.
Prior art FIG. 1 shows a cross-section view of a conventional planar optical waveguide. As depicted in FIG. 1, the planar optical waveguide includes a doped SiO2 glass core 10 formed over a SiO2 bottom cladding layer 12 which is on a silicon substrate 13. A SiO2 top cladding layer 11 covers both the core 10 and the bottom cladding layer 12. As described above, the refractive index of the core 10 is higher than that of the cladding layers 11 and 12.
Consequently, optical signals are confined axially within core 10 and propagate lengthwise through core 10. The SiO2 glass core 10 is typically doped with Ge or P to increase its refractive index. In many types of PLC devices, a large number of cores (e.g., 40 or more) are used to implement complex fiber-optic functions, such as, for example, arrayed waveguide grating multichannel multiplexers and de-multiplexers.
Effective refractive index, which is comprised of core and clad refractive index, control is very critical to the planar lightwave circuit devices. For example, the center wavelength of each channel in an arrayed waveguide grating (AWG) device is directly affected by the refractive index of the core. A deviation of refractive index within 0.0001 will cause the channel center wavelength to vary in the region of 0.1 nm. For a 40 channel AWG operating in the C band (1520 nm˜1565 nm), the channel to channel spacing is only 0.8 nm. Therefore, the effective refractive index has to be accurate to about 0.0003 across the substrate to provide a high quality AWG device.
In addition to effective refractive index control, the performance of AWG devices is critically dependent upon the precise control of the physical path length of each of the comprising waveguides. The physical path length is determined by the resolution of fabrication process, such as photolithography technique. The product of effective refractive index and physical dimension “l” is the optical path length. The optical path length comprises the dominant influence in the performance of an arrayed waveguide grating (AWG) device. The optical function of AWG is categorized as a finite impulse response (FIR) filter. It's optical performance is determined by the amplitude and optical path length of multiple paths. The transfer function of the AWG can be expressed by the amplitude ak and the optical pathlength Lk of the k arrayed waveguide and is shown asH(λ)=Σak*exp(−2πLk/λ)
where H is the transfer function and λ is wavelength of light. The optical spectrum response of the filter, i.e., insertion loss, is a complex conjugate of the transfer function:IL(λ)=−20*log(|H(λ)|2)=−20*log(|Σak*exp(−2πLk/λ)|2)
In addition to the amplitude response of the AWG filter, often referred to as the insertion loss spectrum, the phase response of the transfer function also contributes to the dispersion aspect of the filter function. More specifically, the group delay of the filter is the derivative of the phase response as a function of the wave vector:Φ(λ)=Im(H(λ))where “Im” is the operator of imagery function and “Φ(λ)” is the phase response of the AWG filter.
Using conventional fabrication processes, core refractive index control is limited to about plus or minus 0.0001. Optical path length control (e.g., the length of the particular waveguides) is similarly limited by the performance of fabrication process to about plus or minus 0.01 microns. These fabrication limitations limit the amount of channel isolation that of can be obtained in a dense wavelength division multiplexing (DWDM) application, and thus limit the number of channels that can be implemented.
Thus, what is required is a solution that improves core refractive index control of complex PLC devices incorporating multiple waveguide cores. What is required is a solution that improves optical path length control of complex PLC devices incorporating multiple waveguide cores. What is further required is a solution that precisely optimizes channel isolation of AWG devices. The present invention provides a novel solution to these requirements.